Kabul: A suicide bomber struck outside NATO headquarters in Kabul on Saturday, killing six street children a day after the United States moved to blacklist the Haqqani faction as a terrorist organisation.

The Taliban, leading a decade-long insurgency against NATO troops and the Afghan government, swiftly claimed responsibility, saying the CIA had been the target to avenge US moves to designate as terrorists their most feared faction.

It was the deadliest attack in the fortified capital since Taliban fighters raided a nearby lakeside hotel on June 22, killing at least 18 people.

And the assault defied stepped-up Afghan security measures put in place across Kabul as national leaders commemorated 11 years since the death of Ahmad Shah Massoud, an conic anti-Taliban commander two days before 9/11.

The bombing was also likely to renew questions about stability in Afghanistan as NATO combat troops withdraw from the country before an end-2014 deadline when they are due to transfer responsibility to their Afghan counterparts.

The blast reverberated through Kabul's diplomatic quarter, which is home to many Western embassies, shortly after First Vice President Mohammad Qasim Fahim finished an address to scores of dignitaries at an event mourning Massoud.

"It was a suicide attack that killed six people and wounded five others," interior ministry spokesman Sediq Sediqqi told.

Police officers confirmed the toll. Police spokesman, Hashmat Stanikzai, said the dead and wounded were all street sellers and odd jobs boys aged between 12 to 17.

Street children routinely gather outside NATO headquarters to peddle small trinkets and sweets, looking out for soldiers leaving or getting into the base.

Witnesses at the Emergency Hospital said most of the victims were teenagers or young men.

Hamid, 23, rushed to the casualty department after being told that his 15-year-old brother, Naweed, a hawker selling chewing gum and sweets, had been wounded.

"These are all animals, the Taliban who kill our people everyday. They told me my brother was brought to this hospital. I'm trying to get in to see him," he said.

Gul Agha, 42, said he was searching hospitals in Kabul to find his 19-year-old son.

"Elyas my son is missing. I'm going from hospital to hospital to find him. I've been to two other hospitals already," he said.